The gasification of a biomass, such as a sewage sludge or a woody biomass, in a pyrolysis gasification system and conversion of the biomass, which has been conventionally treated as waste, into resources for effective use has been in demand. For example, manufacturing a carbide by gasifying and carbonizing a sewage sludge in a pyrolysis gasification system, and using the carbide as carbonized fuel for power generation has been proposed and put to practical use.
Examples of carbonization systems that manufacture carbonized fuel from this sewage sludge include a system configured to include a drying process facility, a pyrolysis gasification system (a pyrolysis gasification facility), and an exhaust gas treatment facility (refer to Patent Document 1, example). The drying process facility subjects the sewage sludge (dewatered sludge) to a drying process. The pyrolysis gasification system gasifies the dried sludge through heating and pyrolysis in a low oxygen atmosphere using a pyrolysis gasification furnace to form a carbide, and further subjects the pyrolysis gas to a clean combustion and combustion deodorizing process at a high temperature using a combustion furnace. The exhaust gas treatment facility collects waste heat from a combustion exhaust gas, and subjects the exhaust gas to, for example, desulfurization, demineralization, cooling, and dehumidification.
Further, in this type of carbonization system, the waste heat collected by the exhaust gas treatment facility and the waste heat from the combustion furnace of the pyrolysis gasification system are fed to the drying process facility and the pyrolysis gasification furnace of the pyrolysis gasification system, and utilized as a heat source for drying and carbonizing the sewage sludge. Furthermore, the exhaust gas discharged from the drying process facility is fed to the combustion furnace of the pyrolysis gasification system, and subjected to the clean combustion and combustion deodorizing process at a high temperature along with the pyrolysis gas.
Further, the pyrolysis gasification system is configured to pyrolyze the sewage sludge (the biomass) at a temperature of 300 to 600° C. in the absence of oxygen by means of the pyrolysis gasification furnace, separate the carbide and the pyrolysis gas by means of a solid-gas separation unit arranged side by side with the pyrolysis gasification furnace, feed the separated pyrolysis gas to the combustion furnace through a pipe which constitutes a pyrolysis gas line, and subject the pyrolysis gas to combustion processing.
Meanwhile, in the pyrolysis gasification system thus configured, pyrolysis components such as tar in the pyrolysis gas fed from the pyrolysis gasification furnace to the combustion furnace are polymerized and gas-phase precipitated (condensed) by polycondensation, and adhere to and accumulate on, for example, an inner wall of the pipe which constitutes the pyrolysis gas line, and an impeller of a fan arranged midway on the pipe, as a pyrolysis deposit. It should be noted that the amount of pyrolysis deposit that occurs can be expressed by a linear function of a pyrolysis component concentration of the pyrolysis gas and a reaction time. Then, when the amount of the adhered and accumulated pyrolysis deposit increases as a result of continuous operation, for example, blockage of the pipe, and an increase in fan output and vibration in association with deterioration of impeller balance occur. For this reason, the pyrolysis gasification system conventionally needs to be periodically stopped to clean the interior of the pipe that constitutes the pyrolysis gas line and remove the pyrolysis deposit.
In response, Patent Document 1 describes a method for removing a pyrolysis deposit that includes stopping the operation of the pyrolysis gasification system when fan output exceeds a reference value or at a stage when the pyrolysis gasification system has been operated a certain period of time, and burning and removing an adhered and accumulated pyrolysis deposit by circulating a mixed gas of an inert gas and oxygen in the pipe that constitutes the pyrolysis gas line provided between the pyrolysis gasification furnace and the combustion furnace.
In this method for removing a pyrolysis deposit thus performed after stopping the operation of the pyrolysis gasification system (the method for removing a pyrolysis deposit on the basis of offline decoking), a mixed gas having an oxygen concentration of at least 5 volume % and no greater than 13 volume % is circulated in the pyrolysis gas line at a gas temperature of at least 500° C. This makes it possible to oxidize the pyrolysis deposit by partial combustion, promote a solid-to-gas phase change, and efficiently burn and remove the pyrolysis deposit. Further, in this method for removing a pyrolysis deposit, the oxygen concentration of the mixed gas fed to the pyrolysis gas line of the pyrolysis gasification system thus stopped is 13 volume % or less, thereby preventing runaway combustion of the pyrolysis deposit and the occurrence of explosion by carbon monoxide, hydrogen, methane, or the like that forms as a result of combustion of the pyrolysis deposit.